An unmanned military space plane landed at Kennedy Space Center Sunday with a sonic boom.

The reusable X-37B Orbital Test Vehicle's (OTV-4) return concluded an almost two-year mission in orbit, according to the US Air Force.

The Air Force tweeted shortly after 8am that the vehicle had landed safely, marking the conclusion of its fourth classified mission, which lasted more than 700 days.

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The reusable X-37B Orbital Test Vehicle (pictured) returned Sunday morning, concluding an almost two-year mission in orbit, according to the US Air Force

The Air Force tweeted shortly after 8am that the vehicle had landed safely, marking the conclusion of its fourth classified mission, which lasted more than 700 days

The unmanned X-37B, which resembles a mini space shuttle, landing on a runway used by the now-mothballed craft.

The Boeing-built space plane blasted off in May 2015 from nearby Cape Canaveral Air Force Station aboard an Atlas 5 rocket built by United Launch Alliance, a partnership between Lockheed Martin Corp and Boeing Co.

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The X-37B, one of two in the Air Force fleet, conducted unspecified experiments for more than 700 days while in orbit. It was the fourth and lengthiest mission so far for the secretive program, managed by the Air Force Rapid Capabilities Office.

The Air Force said the orbiters 'perform risk reduction, experimentation and concept-of-operations development for reusable space vehicle technologies.'

However, the exact nature of the mission and its cost is classified.

X-37B TEST SHUTTLE

Contractor: Boeing

Height: 9 feet 6 inches

Length: 29 feet 3 inches

Wingspan: 14 feet, 11 inches

Weight: 11,000 pounds

Power: Gallium Arsenide Solar Cells with Lithium-Ion batteries

Launch Vehicle: Alliance Atlas V (501)

The Secure World Foundation, a nonprofit group promoting the peaceful exploration of space, says the secrecy surrounding the X-37B suggests the presence of intelligence-related hardware being tested or evaluated aboard the craft.

The vehicles are 29 feet long and have a wingspan of 15 feet, making them about one quarter of the size of the National Aeronautics and Space Administration's now-retired space shuttles.

The X-37B, also known as Orbital Test Vehicle, or OTV, first flew in April 2010 and returned after eight months. A second mission launched in March 2011 and lasted 15 months, while a third took flight in December 2012 and returned after 22 months.

Sunday's landing was the X-37B's first in Florida. The three previous landings took place at Vandenberg Air Force Base in California.

The Air Force relocated the program in 2014, taking over two of NASA's former shuttle-processing hangars.

The Air Force intends to launch the fifth X-37B mission from Cape Canaveral Air Force Station, located just south of the Kennedy Space Center, later this year.

Air Force officials have called the X-37B program 'the newest and most advanced re-entry spacecraft.' The program is managed by the Air Force Rapid Capabilities Office

When the plane landed Sunday morning the sonic boom could be heard throughout Central Florida, and some Brevard County residents likely heard them before it even touched down at the Shuttle Landing Facility.

Air Force officials said that several technologies are being tested in the program.

'The primary objectives of the X-37B are twofold; reusable spacecraft technologies for America's future in space and operating experiments which can be returned to, and examined, on Earth,' an official said, according to Click Orlando.

Officials also explained that the X-37B is the first vehicle of its kind able to return experiments to Earth for further inspection and analysis. It is the first with this capability since NASA's Shuttle Orbiter.

The X-37B is able to stay in space longer because of its minimum on-orbit time of 270 days, officials said.

When the plane landed Sunday morning at the Kennedy Space Center (pictured) the sonic boom could be heard throughout Central Florida, and some Brevard County residents likely heard them before it even touched down at the Shuttle Landing Facility

According to the Air Force, this is the fourth successful mission for the project, with the first three landing at Vandenberg Air Force Base in California.